php - 单例模式的一点小疑问

最近在开始一些小项目,接触到单利模式,本着定义类的时候,尽可能的隐藏信息的原则,我对类进行了以下的编写

class openGate{
        private $dbname='mysql:host=localhost;dbname=project';
        private    $username='root';
        private    $password='root';
        private static $key=null;
        private function __construct(){}
        private function __clone(){}
        public static function gateKey(){
            if (self::$key==null) {
                self::$key=new openGate();
                return self::$key->gateWay();
            }
            return self::$key->gateway();
        }
        public function gateWay(){
            return new PDO($this->dbname,$this->username,$this->password);
        }
    }
    $person=openGate::gateKey();
    var_dump($person);

请问一下,这样和以下相比

class openGate{
        private static $key=null;
        private function __construct(){}
        private function __clone(){}
        public static function gateKey(){
            if (self::$key==null) {
                $dbname='mysql:host=localhost;dbname=project';
                $username='root';
                $password='root';
                return self::$key=new PDO($dbname,$username,$password);
            }
            return self::$key;
        }
    }
    $person=openGate::gateKey();
    var_dump($person);

有哪些优缺点,或者是只是单纯的多余呢?